From Valve's Linux blog: "That the Linux version runs faster than the Windows version (270.6) seems a little counter-intuitive, given the greater amount of time we have spent on the Windows version. However, it does speak to the underlying efficiency of the kernel and OpenGL." If it wasn't obvious before, it should be now: Valve has started its marketing campaign for Linux. With the Windows platform in the process of closing itself off, Valve has to look to greener pastures. This is all to motive third parties to get their stuff ready for a possible Linux-powered 'Steambox' - not a console, but a set of generic PC specifications. Remember: the Xbox is the only machine tied to DirectX - OpenGL runs everywhere else, including Windows (the PS3 is an oddball, and has a sort-of Sony-specific FrankenOpenGL). OpenGL simply makes more sense for developers, and now Valve is working very closely with Nvidia, AMD, and Intel to optimise their Linux drivers. Do the math, people.

The thing is most Steam users I believe own L4D2, so they already own it.

If you own it you can play it on any computer (Mac, Windows, Linux) that you has the Steam client installed and you are logged into. I could imagine since most of their customer do not run Linux that usage will be very small at first. However I could imagine it will grow.

There were also rumors about Steam distributing other applications besides games, so perhaps their plan is eventually launch their own distribution or something (based on Ubuntu).